Geoscience Reference
In-Depth Information
Figure 5.1 Structure of
α
-quartz
[5]
.
silicons as nearest neighbors. There are several works on the hydrothermal synthe-
sis of other varieties of silica—like coesite,
β
-quartz, stisshovite, cristobalite, and
tridymite
[7
10]
. However, the scope of this chapter has been restricted to the
growth of large size single crystals of quartz by the hydrothermal technique. In this
respect, it is only the
α
-quartz, which could be grown as large bulk single crystals,
whereas the other varieties of silica have been obtained so far only as fine crystal-
line products, or tiny crystallites.
The
α
-quartz is one of the most extensively studied materials. It is the most popular
and technologically very important among all the hydrothermally grown crystals or
materials. Quartz, as a piezoelectric crystal, has the ability to convert electric waves
into mechanical waves and to reverse the process. Because of this property, quartz is
widely used in filters, timing and frequency control applications, optical fibers, dielec-
tric applications, and so on. In the recent years, quartz “tuning forks” have become
essential for timing functions in electronic watches and in timing circuits for computers
and telecommunications. Quartz crystal is one of the most transparent materials over a
wide range of optical frequencies from UV (ultraviolet) to IR (infrared) regions, and it
has double refraction (birefringence) and optical rotation power. Because of these
properties, the quartz crystal is utilized as an optical device on a large scale. Typical
applications are in optical low pass filters (OLPF) for video cameras and waveplates
for optical pickup.
Table 5.1
gives a wide range of applications for
-quartz.
The principal source of electronic grade natural quartz is Brazil. Today, the
electronic industries are largely inclined to use synthetic quartz, because natural
quartz crystals are generally irregular in shape, automatic cutting is cumbersome,
and the yield is low. Over 3000 tons of quartz are produced annually for a variety of
applications. These applications range from optical components (due to its high
transparency) to precise time and frequency oscillators based on its piezoelectric
properties. The important countries contributing to the world production are the
United States, Japan, China, Korea, Taiwan, Poland, Belgium, France, United
Kingdom, Germany, Russia, Brazil, and so on. At one time, Japan alone produced
more than 50% of the worldwide production, followed by the United States till
recently
[11]
. Now, China has emerged as one of the major contributors to the world
α
